Develop Reference

How to strictly allow exactly ONE char as input using SCANNER reader with reference to my code?

The code below is for a simple calculator with the four basic mathematical operators. It is a working program, it works as expected. However, I have a few questions to understand and improve both my program as well as my understanding of Java. (I have used google but the amount of redundant info confuses me and haven't found any perfect answers on StackOverflow too, though there are dozens of related questions. Believe me, I did tried before posting here).
How can I make sure that the user input is exactly and strictly one char?
here in my program, it accepts more than one character (+-*) but operates on the first char (+) only. I want to make sure more than one character is not accepted as input.
After successful execution of the program, how can I somehow let the user repeat the main method? I mean, a user adds two numbers, gets his answer and he wants to do another calculation, maybe multiply two numbers this time. I can ask the user for yes or no to continue but how do I take him/her back to the beginning? (will a loop work? how?)
A the end of the program I used two methods to output a message. The system.out.print works fine but the JOptionPane method doesn't display the message and the program doesn't terminate (I have commented it out). I would like to understand why?
Is the default case required in the switch? And Am I following the correct code structure? (the arrangements and uses of curly braces)
NB: As I said this calculator works fine and can be used by newbies like myself to better understand the concept as I have commented on every detail. Please understand that I couldn't add everything in the question title due to limits...
package mycalculator;
import javax.swing.JOptionPane;
import java.util.*;
public class MyCalculator{
public static void main (String [] args){
// Let us code a simple calculator//
// Variable type declarations//
char OP;
int firstNum;
int secNum;
// Display an explanation of what this program does//
System.out.println("This is a simple calculator that will do basic
calculations such as :"
+ "\nAddition, Multiplication, Substraction and Division.");
// Create a scanner object to Read user Input.//
Scanner input = new Scanner(System.in);
// Ask user to input any positive number and read it//
System.out.println("Enter Any positive number followed by pressing
ENTER.");
firstNum = input.nextInt();
// Ask user to input/decide his choice operator and read it //
System.out.println("Enter a valid OPERATOR sign followed by pressing
ENTER.");
OP = input.next().charAt(0);
// Loop the below statement till one of the four (+,-,*,/) is entered.//
while(OP != '+' && OP != '-' && OP != '*' && OP != '/'){
System.out.println("Please Re-enter a valid Operator (+,-*,/)");
OP = input.next().charAt(0);}
// Ask user for any second number and read it//
System.out.println("Enter your Second number followed by an ENTER
stroke.");
secNum = input.nextInt();
// Various possible Resolution based on OP value.//
int RSum = firstNum+secNum;
int RSubs= firstNum-secNum;
int RPro = firstNum*secNum;
double DPro = firstNum/secNum;
// Conditional statements for Processing Results based on OP and display.//
switch(OP){
case '+': System.out.println("The Resulting sum is "+ RSum);
break;
case '-': System.out.println("The Resulting sum is "+ RSubs);
break;
case '*': System.out.println("The Resulting Product is "+ RPro);
break;
case '/': System.out.println("The Resulting Divisional product is "+
DPro);
break;
//Maybe the default case is actually not needed but is added for totality//
default : System.out.println("Try Again");
break;
}
// The following code maybe deleted, it is for experimental purpose//
// Just checking if additional statements executes after a program
completes//
System.out.println("Test Message ");
// JOptionPane.showMessageDialog(null, "The Process Ends Here!");
//The test message works fine//
//The JOptionPane statement don't work and program doesn't end. WHY?//
}
}

How can I make sure that the user input is exactly and strictly one
char? here in my program, it accepts more than one character (+-*) but
operates on the first char (+) only. I want to make sure more than one
character is not accepted as input.
If you use console application and Scanner, only thing that you can do is read a String and check its length. In case you use Swing, you could implement KeyPressListener and proceed exactly after user press a button (but not for console application).
After successful execution of the program, how can I somehow let the
user repeat the main method? I mean, a user adds two numbers, gets his
answer and he wants to do another calculation, maybe multiply two
numbers this time. I can ask the user for yes or no to continue but
how do I take him/her back to the beginning? (will a loop work? how?)
You can't repeat main method. In Java main method is been executing only once. To repeate your code, you could wrap whole main method content to the infinite loop or move the content to the separate method and call it from the loop in the main method.
A the end of the program I used two methods to output a message. The
system.out.print works fine but the JOptionPane method doesn't display
the message and the program doesn't terminate (I have commented it
out). I would like to understand why?
JOptionPane works only for graphic application (Swing/AWT). This is not available in console. You have only standard input and output there.
Is the default case required in the switch? And Am I following the
correct code structure? (the arrangements and uses of curly braces)
No, default case is optional by JVM syntax. I remember, that e.g. in C++ there was reccomendation to place it (event empty), to exclude side effects of compilators. I do not know, is there such reccomendation in Java, but when I use switch, I prefer to always add it to exclude logical problem (but this is definetly optional according to syntax case). You use switch correctly.
public static void main(String[] args) {
System.out.println("This is a simple calculator that will do basic calculations such as :"
+ "\nAddition (+)"
+ "\nMultiplication (*)"
+ "\nSubtraction (-)"
+ "\nDivision (/)");
System.out.println();
try (Scanner scan = new Scanner(System.in)) {
while (true) {
System.out.println("Enter Any positive number followed by pressing ENTER.");
int first = scan.nextInt();
System.out.println("Enter a valid OPERATOR (+,*,-,/) sign followed by pressing ENTER.");
String operator = scan.next();
while (operator.length() != 1 || !"+*-/".contains(operator)) {
System.out.println("Please Re-enter a valid Operator (+,-*,/)");
operator = scan.next();
}
scan.nextLine();
System.out.println("Enter your Second number followed by an ENTER stroke.");
int second = scan.nextInt();
if ("+".equals(operator))
System.out.println("The Resulting sum is " + (first + second));
else if ("*".equals(operator))
System.out.println("The Resulting mul is " + (first * second));
else if ("-".equals(operator))
System.out.println("The Resulting sub is " + (first - second));
else if ("/".equals(operator))
System.out.println("The Resulting div is " + ((double)first / second));
System.out.println();
System.out.println("Do you want to exit ('y' to exit)?");
if ("y".equals(scan.next()))
return;
System.out.println();
}
}
}

1) you can check size of string input.next() .If it is one then continue else again prompt for operator choice .
2)I would suggest better create a different method and put all logic in it and call it the number of time you want or call infinite number of times.
4)Should switch statements always contain a default clause?

Check String is in a range of numbers Java

Trying to design a simple lottery program. Everything works except checking if the numbers entered are between 1 to 59.
Exercise says the numbers must be stored in a String variable.
so
if(num<0 || num>59) //wont work for me
Tried making another variable
int numConverted = Integer.parseInt(num)
We haven't covered converting String to int in class though so I don't think this is what expected. Got confused trying that way anyway so probably this is wrong.
Here is the code I have currently.
{
Scanner scan = new Scanner(System.in);
String num=""; //num variable is empty untill user inputs numbers
for(int i =0; i<6; i++)
{
System.out.println("Enter your number between 1-59");
num = num +" "+ scan.nextLine();
}
System.out.println("Ticket printed £2. Your numbers are " + num);
}

In your posted code it's obvious that you want the User to supply 6 specific numerical values. These values are appended to the String variable named num (space delimited). You need to obviously do a few things here:
1) Make sure the value supplied by the user is indeed a numerical value;
2) Make sure the numerical values supplied fall within the minimum and maximum scope of the lottery itself (which you have stated is: 1 to 59);
3) Make sure the number entered by the User hasn't been supplied already.
You've been tasked to store the entered values into a String data type variable and that is all fine but at some point you want to carry out value comparisons to make sure that all the entered values actually play within the limits of the lottery.
When the User completes his/her entries, you end up with a space delimited string held in the num string variable. You now need to make sure that these values entered are indeed....numbers from 1 to 59 and none contain alpha characters.
In my opinion (and this is only because you need to store entered values into a String variable), it's best to use your String variable to gather User input, then test the input to make sure it is indeed a string representation of an actual integer number. Once this is established then we test to make sure if falls within the value min/max limits (1-59). Now we need to test to make sure the number entered hasn't already been entered before for this ticket.
Of course with each test described above, if one fails then the User should be prompted to re-enter a proper value. You can do this by utilizing a while loop. Plenty examples of this in StackOverflow but here's a quick example:
Scanner scan = new Scanner(System.in);
String ticketNumbers = "";
for(int i = 0; i < 6; i++) {
Boolean isOK = false;
while (!isOK) {
System.out.println("\nPlease enter your desired 6 ticket numbers:\n"
+ "(from 1 to 59 only)");
String num = scan.nextLine();
//Is the string entered an actual integer number?
//We use the String.matches() method for this with
//a regular expression.
if(!num.matches("\\d+")) {
System.out.println("You must supply a numerical value! "
+ "Try Again...");
continue;
}
if (ticketNumbers.contains(num + " ")) {
System.out.println("The number you supplied has already been chosen!"
+ " Try Again...");
continue;
}
if (Integer.parseInt(num) >= 1 && Integer.parseInt(num) <= 59) {
ticketNumbers+= num + " ";
isOK = true;
}
else {
System.out.println("The number you supply must be from "
+ "1 to 59! Try Again...");
}
}
}
System.out.println("Ticket printed £2. Your numbers are " + ticketNumbers);

How about -
if(Integer.parseInt(num) < 0 || Integer.parseInt(num) > 59)
This should work, place it after the input.
If it works, please mark this as correct, I need the rep!!!

Easy way would be add available numbers (suppose it wont grow more than 60. You can use a loop to add to this as well)
String numbers[] = {"1","2","3", "..."};
Then inside the loop
Arrays.asList(numbers).contains(num);
You can remove prefixing zero in order avoid conflicts with values like '02'
Here everything is String related.

If you don't want to explicitly convert to int, you could use a regular expression.
if (num.matches("[1-5]?[0-9]")) {
...
This checks whether the String consists of (1) maybe a digit from 1 to 5, followed by (2) definitely a digit from 0 to 9. That'll match any number in the range 0-59.
If you've got a whole series of numbers separated by spaces, you could expand this to cover a whole series like this.
if (num.matches("([1-5]?[0-9]\\s+)*[1-5]?[0-9]")) {
This matches any number of repetitions (including zero) of "a number followed by spaces", followed by a single repetition without a space. The "\\s" means "any whitespace character", the "+" after it means "one or more of what precedes", and the "*" means "zero more of what precedes" - which in this case is the term in parentheses.

Oh I see what you are trying to do
This is what you want
Scanner scan = new Scanner(System.in);
String allNums = "";
for(int i =0; i<6; i++)
{
System.out.println("Enter your number between 1-59");
int num = scan.nextInt();//Take the number in as an int
if(num >0 && num < 59)//Check if it is in range
{
allNums += num + " ";//if it is add it to a string
}
else
{
System.out.println("Number not in range");
i--;//go back one iteration if its not in range
}
}
System.out.println("Ticket printed £2. Your numbers are " + allNums);

Int will not work properly

I am very new to coding, have only completed a few hours of YouTube videos to learn thus far. I am trying to complete a practice code and am facing some trouble.
I have attached a part of the code below. When I am entering value in (10,12,14, and 16) the code is still responding with "Wrong Response". In addition to this the following line is not properly functioning. It is not giving me the option to select a crust type. Please let me know if anyone has any suggestions.
Crust problem:
System.out.println("What type of crust would you like? ");
System.out.print("(H)and-tossed, (T)hin-crust, or (D)eep-dish: ");
crust = keyboard.nextLine();
Int Value problem:
if ( size.equals(" 10 ")) {
pizzaPrice = SM_Price;
} else if ( size.equals(" 12 ")) {
pizzaPrice = MED_Price;
} else if ( size.equals(" 14 ")) {
pizzaPrice = LG_Price;
} else if (size.equals(" 16 ")) {
pizzaPrice = XL_Price;
}
else { System.out.println("Wrong repsonse. ");
Thank you.

So, keep in mind that the constants 16, "16", " 16 " are all different things and non-equal to each other.
The other thing is that you need to show us what your types are. As Java is statically typed, that type information can actually help determine the behavior that you get.

if size is an int, then using size against string will not work. i.e size.equals(" 10 ") will not be the same as size.equals(10).
Also will not be the same as size.equals("10") which is different from " 10 "

You haven't defined size anywhere, so it's impossible to say.
I suspect your problem however is either that size is a different type that what you're comparing against using equals, or it's because you have spaces before and after your string numbers: " 12 ".

Please refer to User Input not working with keyboard.nextLine() and String (Java) for your nextLine() problem.
As to your other question one needs to know of what type you declared size to be.
If you declared it as String using String size = "10"; for example, then you are quite close:
Just change " 10 " in line 1 to "10" (without whitespace, as "10".equals(" 10 ") == false) and go for the small pizza.

Java calculator

I want to take a number the user enters and multiply it by a couple of different numbers I set but I can't figure out how to declare an integer and get it to play nicely with whatever number the user enters. I've tried writing
System.out.println("If you take" + stepsOne * firstNum + "steps in a 10 second interval, you could potentially achieve...");
and just about every variation I can think of but I keep getting error messages. Apparently javac hates me for some reason. :/
Also, whenever the greeting displays, there's no space between the data the user enters and my system.out.println stuff so if they enter the name "George" it comes out as "HelloGeorge". Not as big of an issue as the above but if you know how to fix that then have at it. :)
Here's my code:
import java.util.Scanner;
public class Calculator
{
public static void main(String[] args)
{
Scanner userInputScanner = new Scanner(System.in);
int firstNum;
firstNum = 6;
System.out.println ("Hello, my name is Bob. What is your name?");
String userName = userInputScanner.nextLine();
System.out.println ("Hello," + userName + ". How many steps do you take in a ten second interval?");
String stepsOne = userInputScanner.nextLine();
System.out.println("If you take" + stepsOne + "steps in a 10 second interval, you could potentially achieve...");
System.out.println ("Number of steps per minute:");
System.out.println ("Number of steps per hour:");
}
}

There is a crucial difference between the string 123 and the integer 123 - the first is merely a sequence of characters, which have no mathematical meaning. You need to parse the string, meaning that you must construct an integer based on the characters of the string. There's a built-in method for this (although it's very interesting to do it yourself too): Integer.parseInt().
String stepsOneString = userInputScanner.nextLine();
int stepsOne = Integer.parseInt(stepsOneString);
Now, stepsOne is an integer on which you can perform mathematical operations. However, Integer.parseInt() throws an exception that you'll need to handle. At this point in your course, I'm guessing that you're expected to use the built-in conversion capabilities of the Scanner:
int stepsOne = userInputScanner.nextInt();
This will essentially perform the two above steps for you.
I strongly recommend that you read up on the subject of data types, which will explain the above concepts.

Once you have a string representing a number, you can use Integer.parseInt() to get an integer from it.
String stringSteps = userInputScanner.nextLine();
int steps = Integer.parseInt(stringSteps);
Here is a complete program with improved spacing.
import java.util.Scanner;
public class Calculator {
public static void main(String[] args) {
Scanner userInputScanner = new Scanner(System.in);
System.out.println ("Hello, my name is Bob. What is your name?");
String userName = userInputScanner.nextLine(); //Asks user name
System.out.println("Hello, " + userName +
". How many steps do you take in a ten second interval?");
String stringSteps = userInputScanner.nextLine();
int steps = Integer.parseInt(stringSteps);
System.out.println("If you take " + steps +
" steps in a 10 second interval, you could potentially achieve...");
System.out.println ("Number of steps per minute: " + (6 * steps));
System.out.println ("Number of steps per hour: " + (60 * 6 * steps));
}
}

Basically, I just want to take a number the user enters and multiply it by a couple of different numbers I set but I can't figure out how to declare an integer and get it to play nicely with whatever number the user enters. I've tried typing
First problem is stepsOne is String, so it can't be used to perform arithmetic on (at least not they type you're trying). So you need to convert it to a int
int stepsOneInt = Integer.parseInt(stepsOne);
nb: This will throw a NumberFormatException if the String can't be converted to a int value, so beware of that
Now you can use it to perform other arithmetic tasks
System.out.println("If you take " + (stepsOneInt * firstNum) + " steps in a 10 second interval, you could potentially achieve...");
Also, whenever the greeting displays in javac, there's no space between the data the user enters and my system.out.println stuff so if they enter the name "George" it comes out as "HelloGeorge". Not as big of an issue as the above but if you know how to fix that then have at it. :)
That's because you don't add any spaces before you print the value
System.out.println ("Hello, " + userName + ". How many steps do you take in a ten second interval?");

Not sure to understand your issue...
You can't do stepsOne*firstNum as stepsOne is a String.
If stepsOne must be an int, just do this:
int stepsOne = userInputScanner.nextLine(); //Asks user a nb

How to display the middle digit in java

I am trying to get java to display the middle digit of a 1-4 digit integer, and if the integer has an even number of digits i would get it to display that there is no middle digit.
I can get the program to take get the integer from the user but am pretty clueless as how to get it to pick out the middle digit or differentiate between different lengths of integer.
thanks

Hint: Convert the integer to a String.
Java int to String - Integer.toString(i) vs new Integer(i).toString()
You may also find the methods String.length and String.charAt useful.

This sounds like it might be homework, so I will stay away from giving an exact answer. This is much more about how to display characters than it is about integers. Think about how you might do this if the questions was to display the middle letter of a word.

Something like this?
import java.util.Scanner;
public class Main {
public static void main(String args[]) {
Scanner s = new Scanner(System.in);
System.out.print("Enter an integer: ");
while (!s.hasNextInt()) {
s.next();
System.out.println("Please enter an integer.");
}
String intStr = "" + s.nextInt();
int len = intStr.length();
if (len % 2 == 0)
System.out.println("Integer has even number of digits.");
else
System.out.println("Middle digit: " + intStr.charAt(len / 2));
}
}
Uhm, I realized I might just have done someones homework... :-/ I usually try to avoid it. I'll blame the OP for not being clear in this case